Powder metallurgy represents an important branch of materials processing technologies. Typically, such operations entail the generation of new phases or structures from a blend of powdered precursors, which consist of pure elements or chemical compounds. The constituents may be in any of the three states of matter, solid, liquid, or gas. The blending method, its complexity, and extent may vary from a simple, quick shaking operation to long periods of careful and precise atomic level mixing of the constituent. Powder metallurgy also encompasses monolithic powder processing, such as that occurring during high-energy milling, wherein the objective is not the creation of a new phase, but the reduction of the initial particulates into fine or ultrafine (i.e., milli-, micro-, or nanoscale precursors and the concomitant modification of their internal structure.
It is important to realize that while powder metallurgy is usually associated with metals-based research, the handling and processing of powders are commonplace in many industries. Multiple examples can be found in the food processing, agricultural, and pharmaceutical industries, not only at the laboratory scale, but also on larger scales. Various names exist for identifying such operations. These include terminology such as blending, mixing, milling, grinding, alloying, or mechanical alloying, etc.
During all of these processes, energy is imparted to the powdered material. Depending on the desired level of mixing, the amount of energy deposited into the mixture varies by orders of magnitude. That is, the resultant blend has a significant, and usually undetermined, amount of stored potential energy. The intention of this processing methodology is the improved physico-chemical and mechanical properties of the powdered materials. The improvements in physical and chemical properties may translate into greater mechanical strength, ductility, or chemical reactivity.
Parasitic heating is an unintended side effect of this processing. Imparted energy may be dissipated in the heating of the powdered material as well as the surrounding medium. That is, there will be an increase in the temperature of the milled powder as well as a corresponding increase in the temperature of the surrounding media.
Temperature can easily be controlled but heat from a chemical reaction cannot. The primary source of the increase in reactivity, or the ability to quickly release stored potential energy (sometimes explosively), is associated with the greatly reduced particle size (which translates into a large surface area). Thus, the finer the resultant powdered material, the greater its reactivity. The greater reactivity may be harnessed with the intended exposure of the powdered materials to other chemical substances. However, the unintentional, uncontrolled, or untimely exposure to other substances needs to be avoided. In some situations, when the other substance is simply the ambient atmosphere, catastrophic outcomes could result, if extreme vigilance was not practiced.
The handling of the powders during processing, storage, loading, unloading, and post-processing is known to those skilled in the art. For example, processing of the powdered material is carried out in hermetically sealed vessels or containers, loaded and unloaded in a protective environment, and stored in a device such as an inert-gas glove box apparatus. Glove boxes are designed to store powdered or reactive materials in a low oxygen and low later content environment. Typically, the ambient atmosphere is evacuated and replaced by an inert atmosphere; after which, the levels of reactive gases are carefully monitored.
Such an apparatus not only offers access to the controlled inert environment, but also offers a stand-off distance and containment for protecting the operator, if something unexpected were to happen. While a glove-box apparatus eliminates most of the concerns associated with storage, it does not address how the operator is to unload the contents of the milling container or vessel. Thus, there is a need for a device that allows for the safe opening and sealing of such containers and vessels.
In the past, there have been documented instances where the opening of such vessels after processing the material unknowingly resulted in a forceful, extremely rapid, explosive release of the contents severely injuring the operator. The explosive forces were directly attributed to a single or multiple chemical reaction(s) between the contents of the vessel and the ambient atmosphere. The freshly exposed, nascent powdered material surfaces are highly susceptible and will readily react with the components of air (oxygen or nitrogen).
The contents of the vessels may consist of the powdered material that is being processed, the milling media, and processing agents. The milling media, generally consisting of high-strength, high-hardness steel or ceramic spheres (e.g., ball bearings), which are designed to facilitate the breakdown and reduction of the particulates; they usually remain inert and do not contribute to enhancing the properties of the product material. In contrast, the processing agents are designed to enhance or retard the milling process, or potentially alter the reaction products. As such, these may be liquids, solids, or gases.
Thus, there is a need for a device that allows for the safe depressurization of such containers and vessels. Therefore, the increased risks to the operator come from the combination of these several factors: dramatically increased particulate surface area which lowers the auto-ignition threshold or barrier, leading to much more rapid reaction kinetics; presence of process agents which may have decomposed; and gaseous byproducts under pressure.
All of these factors can readily contribute to the instantaneous decompression and simultaneous ignition of fuel contained in the vessel. As few as one to as many as one hundred ball bearings can be used in conjunction with milling; and, the size of the bearings may range in size from 1/16 to 1 inch in diameter. During an explosive decompression, the ball bearings become projectiles or shrapnel, with a potential to kill or maim the operator. Given the intrinsic nature of the powdered material or materials being milled, and the type of processing control agent, it is difficult to anticipate all possible reactions. Thus, there is a need for a simple, reliable, safe, and routinely useable method and device to vent, decompress, and open milling vessels in a safe controlled manner without risk or harm to the operator.